The present invention relates to a process and device for filling, with a corrosion protective hardenable filler material, the ring-shaped gaps which form between the tubes and fins on assembling tube and fin type heat exchangers.
A so-called tube and fin type heat exchanger is commonly used for liquid-to-gas or gas-to-liquid heat transfer. This kind of equipment is used mostly for transferring heat from a liquid to the air, (for cooling liquids or heating the air), or from the air to a liquid, (air conditioners, evaporators). To compensate for the poor heat transfer to the air, the tubes carrying the liquid in such heat exchangers are fitted with so-called fins or lamellae on the outside to increase the heat emitting surface in contact with the air.
There are various ways of manufacturing such heat exchangers. One design which is produced widely employs small disc-shaped fins made of sheet material stacked in series on centrally positioned tubes. Particularly suitable for this purpose are sheet materials which exhibit high thermal conductivity as this allows the fins to be relatively thin. In the case of one well known design the individual fins of a plurality of tubes are in the form of thin sheets i.e. lamellae or fins penetrated by a plurality of tubes. These lamellae which have a large surface area are provided with a number of openings or holes corresponding to the number of tubes. This way a compact heat exchanger can be made up of tubes and lamellae.
The quality of the joint between the lamellae, or fins as they are usually called, and the tubes carrying the fluid is extremely important as the conduction of heat into the fins and therefore the efficiency of the heat exchanger depends on this.
A proven method of joining tube and fin, and one which is used often as it is suitable for large scale production, is clamping such that a so-called integral collar which is formed out of the fin itself surrounds the tube. This collar serves, at the same time, to conduct heat into the fin and to fix exactly the space between that fin and the next one. The tight fit of the collar onto the tube is achieved by expanding the tube after the assembly of the heat exchanger.
A ring-shaped gap forms at the place of contact between one fin and the flanged integral collar of the next fin. This is unavoidable as it is impossible to reduce the radius of the bent-over edge of the collar to such an extent that a sharp edge is produced.
Under unfavorable atmospheric conditions moisture can penetrate this ring-shaped gap, and substances which promote corrosion can gather there. Such conditions favor crevice corrosion in the relatively small gaps. This, as is well known, can result in intensive corrosive attack of the tubes and the fins. The localized attack due to crevice corrosion can in time lead to perforation of the tube, which practically amounts to the destruction of the heat exchanger.
There are various known methods for protecting heat exchangers from this kind of corrosion. The tubes and the fins can be coated with a metallic cladding, be provided with oxide layers by chemical processes or by anodizing, or they can be given an organic protective coating. The costs involved with these processes are however considerable and in general lead to a marked increase in the cost of producing such units. In the case of organic coatings, because of the reduced thermal conductivity, these present a barrier to heat transfer from the tube to the fin. Consequently, even a relatively thin layer results in a significant reduction in the efficiency of the heat exchanger. This then has to be compensated for by a corresponding increase in the surface area of the fins.
The disadvantage of poor heat transfer can be avoided by coating the tubes with a layer of substance which is still fluid or viscous during assembly of the heat exchanger. However, this results in a new difficulty in that, on inserting the tube, the coating material at the openings in the fins is wiped away in the direction of insertion. Consequently, the complete filling of all ring-shaped gaps, which is absolutely necessary for effective corrosion protection (especially in heat exchangers with long tubes), is not assured.
It is therefore an object of the present invention to develop a process and device which allow the ring-shaped gaps to be filled in a simple manner with a corrosion protective mass without impairing the heat transfer between pipe and fins. The process and the device should also be applicable for the assembly of heat exchangers using long tubes.